The bonding of natural or synthetic rubber to various substrates is well known and is utilized in many industries involving devices which require the interface of rubber and structural parts or components. In order to bond a natural or synthetic rubber to a substrate, an appropriate adhesive is typically applied between the rubber and the substrate under heat and pressure such that the curing of the rubber and the cross-linking or curing of the adhesive can occur simultaneously. Once the rubber has fully cured, any excess rubber must be discarded since fully cured rubber cannot be remelted and used in another application. Therefore, substantial waste is involved in preparing many rubber-bonded parts and components for use in industry.
Thermoplastic elastomers have been developed in order to alleviate this waste problem associated with natural and synthetic rubbers. Thermoplastic elastomers, unlike natural or synthetic rubbers, can be remelted and recycled so as to avoid substantial waste. Although thermoplastic elastomers do not have the strength or other mechanical characteristics required in highly demanding applications, thermoplastic elastomers are a viable alternative for numerous less demanding applications previously utilizing natural or synthetic rubber. One type of thermoplastic elastomer is comprised of cured rubber (EPDM) particles dispersed within a polymer matrix such as a polypropylene matrix and an example of such a commercially available thermoplastic elastomer is the elastomer sold under the trade name SANTOPRENE by Monsanto.
As in the case of natural or synthetic rubbers, thermoplastic elastomers require appropriate adhesives for bonding to various substrates in order to create a bonded part useful in a typical application. Various adhesives and methods have been previously developed for bonding thermoplastic elastomers. For example, U.S. Pat. No. 4,720,398 discloses a process for improving the adhesion of synthetic yarns, cords or fabrics to thermoplastic elastomers by applying a certain adhesive composition to the yarns, cords or fabrics followed by drying of the treated article. The adhesive composition consists of an aqueous solution or dispersion of a polyurethane containing ionic groups, capped isocyanate groups, and groups with a Zerewitinoff-active H-atom.
U.S. Pat. No. 4,652,475 discloses an adhesive formulation for bonding a thermoplastic material to an elastomeric material under heat and pressure. The adhesive formulation contains a blend of thermoplastic and elastomeric materials which incorporates a formaldehyde donor/acceptor system such as resorcinol and hexamethoxymethylmelamine. The blend of thermoplastic and elastomeric materials are selected depending on the type of materials to be bonded.
A new class of melt processible thermoplastic elastomers based on compatible blends of an ethylene copolymer and vinyl or vinylidene halide polymer have been recently developed which are reported to have excellent ozone, weathering, oil and heat resistance. These new types of polymer blend-based thermoplastic elastomers are described in, for example, U.S. Pat. No. 4,613,533 and are particularly effective in applications involving seals, weatherstrips, hoses, wires and cables.
Since these newly developed weather resistant thermoplastic elastomers are based on chemistry substantially different from the chemistry involved in the traditional rubber particle/polymer matrix thermoplastic elastomers, the adhesives developed for use with the prior thermoplastic elastomers have been found not to form effective, heat-and water-resistant bonds with the newly developed thermoplastic elastomers. In order for these new thermoplastic elastomers to be properly utilized in appropriate applications, an adhesive must be developed which will provide for an effective, heat- and water-resistant bond between the thermoplastic elastomer and a desired substrate.